. Bulletin of the Department of Geology. Geology. Fig. 4. Irregular contact of Franciscan sandstone and shale with slabs of shale in sandstone In certain cases isolated lenses of shale ten to fifteen feet long may show a peculiar disruption as indicated in figure 5. No indica-. Fig. 5. Broken lens of shale in Franciscan sandstone tion of faulting or shearing can be seen in the sandstone between the blocks; it is as massive there as elsewhere. The only conclusion the observer can draw is that the shale must have been first hardened and then broken up by some disturbance which occurred at a time
. Bulletin of the Department of Geology. Geology. Fig. 4. Irregular contact of Franciscan sandstone and shale with slabs of shale in sandstone In certain cases isolated lenses of shale ten to fifteen feet long may show a peculiar disruption as indicated in figure 5. No indica-. Fig. 5. Broken lens of shale in Franciscan sandstone tion of faulting or shearing can be seen in the sandstone between the blocks; it is as massive there as elsewhere. The only conclusion the observer can draw is that the shale must have been first hardened and then broken up by some disturbance which occurred at a time when the sand, in which it was embedded, was still unconsolidated. Veins The Franciscan sandstone is in many localities cut by veins of quartz and ealcite, most of them small and many of them very irregu- lar. Their general character is indicated in the photographs (plates 1b and 2a). At Point Richmond some peculiar veins of quartz were found. The walls of these veins have been impregnated with silica in unusual amount, so that the resiilting rock bordering the vein resembles green chert, save that it contains sand grains. Some of the veins are very narrow, and show very little or no quartz, yet there is an impregna- tion of the wall of the fissure for an inch or more on each side. A few of them show very little evidence of an original fissure, looking like dykes of cherty rock running through the sandstone. At this locality there are great numbers of these veins with silicified walls and irregu- lar branching courses, inclined to the Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original University of California, Berkeley. Dept. of Geology. Berkeley : The University Press
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